Laminate composite materials are becoming increasingly common in the constructions of large aircraft. Typical laminate composite materials are composed of layered resin bonded graphite textiles. Like any material disposed along the exterior of an aircraft, laminate composite materials are subject to damages during the service life of an aircraft. In-flight collisions with birds and air-borne debris, and ground collisions involving loading and maintenance vehicles and equipment cause visible impact sites along the exterior of an aircraft. Assessments and repairs of laminate composite materials represent significant challenges with regard to efficiencies in time, cost, and training. Ground maintenance crews may be able to recognize impact sites along aircraft exteriors, but are typically not sufficiently trained or equipped to assess damages that may be associated with impact sites.
Damages within a composite aircraft component tend to initiate at impact sites and propagate into structures in expanding cone patterns. Unfortunately, visible impact sites that represent mere superficial markings are not easily distinguished by visual inspection from those overlying significant internal damages. Non-destructive inspection (NDI) devices are available so that inspections can reveal hidden sub-surface damages. However, typical available technologies require considerable training and experience.
For example, a bond-testing apparatus according to an example of prior art is disclosed in the U.S. Pat. No. 4,215,583 issued to Botsco et al. on Aug. 5, 1980, which patent is incorporated herein by this reference. This patent describes a sonic energy probe that receives a sinusoidal reference signal from an oscillator and develops an output signal that contains phase and amplitude data. The phase and amplitude data contained by the output signal developed by the probe are affected by properties of an inspected structure coupled to the probe. When the probe is disposed against a layered structure having a disbond, the phase difference between the reference signal and the output signal tends to increase with the depth of the disbond. The amplitude of the output signal tends to decrease with the depth of the disbond. Thus, when the amplitude and phase difference are related respectively to radius and angle in a two-dimensional polar coordinate display called a complex impedance plane display, typical data points fall along a spiral pattern. Shallow disbonds are represented in outer portions of the spiral pattern, and deeper disbonds are represented in inner portions of the spiral pattern, for example, as shown in FIG. 3 of the above-identified patent to Botsco et al. While the position of a data point in a complex impedance plane display may provide information about the condition of a structure under inspection, a high degree of training is needed for understanding and using the described apparatus. Thus, like other available NDI approaches, the technology described in the patent to Botsco et al. appears to be in the practice domain of highly trained specialists.
Despite the complexities of aircraft inspection technologies, many of the challenges faced in commercial aviation can be understood at a level where basic human safety, customer satisfaction, and economic feasibilities are the critical issues. A typical scenario faced by commercial airlines occurs when a ground maintenance crew member spots an impact site along the exterior of an aircraft as the craft is serviced between flights. A decision must be made as to whether the aircraft should be permitted to fly or should be grounded for thorough inspections, damage assessments, and repairs if necessary. Consequences can be severe when such a decision is poorly made. Both safety and commercial viability must be preserved. Thus an aircraft with significant subsurface damages along a wing or other structure should be grounded, and an aircraft having mere surface markings but no structural damages should be dispatched for flight. Trained NDI specialists and the complex equipment they may need to deploy are not typically immediately available at commercial aviation facilities. If specialists are to be summoned every time an impact site along the exterior of an aircraft is noticed, flight delays will occur, and passengers may need to be re-routed and possibly accommodated with hotel rooms and meals.
Thus, it would be advantageous to provide devices and methods for inspecting structures by modes requiring minimal training and interpretive expertise. Simplified methods in inspecting layered structures are needed. A need exists for rapid results providing go and no-go indications in NDI inspections.